1. Field of the Invention
The present invention relates to methods and apparatus employing inert gases injected into the lower level of sloping underground oil bearing formations as a driving mechanism and water injected into the upper level of the formations as a gas blocking mechanism for increasing and extending the production of oil from underground formations nearly depleted of natural gas as a driving mechanism.
The present invention also generally relates to methods and apparatus for enhanced oil production. More specifically, the invention relates to methods and apparatus employing underground heating in reservoirs for production of highly viscous crude oils in many oil formations where the dissolved gases (commonly referred to as “light ends” in the Industry) are nearly or completely depleted, and for production of viscous, e.g. paraffin-based, crude oils from reservoirs where, e.g., the entrained paraffin precipitates because of a drop in temperature or pressure and blocks the oil passageways (permeability) in the formations near production wells.
2. Description of Related Art
During geologic times marine animal and vegetable remains collected in ocean basins and were covered by the accumulation of eroded sand and sediment. Over millions of years the organic matter in those saltwater basins changed to what would become oil and gas. The weight of the layers of material that accumulated on top of the sand beds and the high density of the saltwater caused a high pressure to form in the oil and gas basins. Seawater flowing in subterranean strata and other natural forces added to this pressure and caused the oil and gas to flow upwards out of the buried basins. The oil and gas then migrated with the flowing saltwater in the permeable layers of material below the impervious layer serving as a cap rock until captured by anticlines, faults, stratigraphic traps, and other subsurface formations. Similar oil and gas reservoirs are found universally.
Along the coast of the Gulf of Mexico and other areas large bodies of salt penetrated the strata from far below the surface to create domes that could even be seen above the surface in many places. The actions of the salt left porous layers of rock turned upward against the impervious salt, formed pockets in the cap of the domes, and caused faults in the strata above or surrounding the domes to trap the migrating oil and gas. The origin of the salt has yet to be fully understood. Some believe that alone the Gulf Coast of the United States the salt may have originated from the thick horizontal layer of salt that starts on-shore near the northern Texas Coast and extends out for many miles below the waters of the Gulf of Mexico just off the Louisiana Coast. A similar origin is believed in other areas. Oil production on-shore along the Gulf Coast is often around the salt domes as well as many other formations. Similar oil and gas reservoirs are found universally.
In the early part of the industry, before the technological advancements in exploration and drilling that exist today, oil production was from wells drilled into shallow formations. Methane gas above and entrained in the oil maintained the underground pressure and displaced the oil up the wells to the surface. The gas in those earlier fields has long been taken off to provide fuel for homes and industry. Soon after came the installation of the familiar pumps (called “pump jacks”) towering above the ground with the cyclic movement of the giant rocking arms as they lift the oil to the surface. Water and steam pumped down into the oil-bearing formations under ground as a driving mechanism (water and steam flooding) has received a limited amount of success for extraction of additional oil from certain fields. Many of those fields are now mostly depleted of the oil considered to be recoverable. However, it is well understood by those in the industry that in most oilfields more oil remains in oil formations than the amount removed by the previous technology available, perhaps enough to greatly reduce the United States' dependency on foreign oil for some time in the future if production can be recovered. If a new method is successfully demonstrated, the production from existing fields could be almost immediate and at relatively low cost because the location of the fields are known, the formations from which the oil is produced is well understood, and many of the abandoned wells are already in place with minimum effort required for placing them back into production.
The method employed in some aspects of this invention is to use fluids such as inert gases produced by the combustion of methane or propane gases as a driving mechanism. The products of combustion are also generally referred to as “flue gases.” There have been a number of attempts to extend oil production in oilfields considered to be depleted of the readily recoverable oil by injection of inert gases into the oil bearing formations as a driving mechanism that have failed. A second problem experienced in the attempts at inert gas injection was the corrosive effects of flue gases on the equipment and piping both above and below ground. The present invention overcomes the deficiencies of previous methods and apparatus by removing the corrosive contaminants in the flue gases and controlling the direction of flow of the inert gases once injected into the underground formations. The key to the success of extending oil production by inert gas injection in a formation considered depleted of recoverable oil is the addition of a method of controlling the flow path or direction the gases have a tendency to take. The general approach over an entire production zone is to inject the inert gases into the lower level of the inclined oil sand (down dip) to drive the oil up the formations and prevent the gases from escaping by pumping water into the upper part of the oil sand (up dip) to drive the oil in a downward flow to intercept the oil being driven upward by the injected inert gases. The heavier water will block most of the gases from overrunning the oil and escaping out of the production zone.
Injecting the lighter gases through selected injection wells at the lower end of a formation and the heavier compatible water from selected wells in the upper end of that formation will increase the pressure in the formation between the injection points and drive the oil to selected production wells positioned between the two levels of injection to collect the oil and bring it to the surface. The compressible inert gases will maintain a higher formation pressure between the injection wells and keep the oil flowing to the production wells for a period of time after the gas injection is temporarily discontinued. In addition, apparatus designed to reduce costs of oil recovery have been incorporated into the oil production system including small and new crude oil production pumps to replace the large and expensive pump jacks currently used and make it economically feasible to produce even one-quarter barrel of oil per day from a well and a fuel gas generator to extract natural gas from the crude oil under production for operation of the internal combustion engines used to power compressors and electrical generators in the production field.
The inert gases are produced by powering a compressor with an internal combustion engine in the production field or obtained from the combustion flue of a nearby industry. Air is added to the combustion process, and as a result for one theoretical cubic foot (ft3) of methane fuel the volume of combustion products produced include 1 ft3 of carbon dioxide (CO2), 2 ft3 of water vapor (H2O), and 7.55 ft3 of nitrogen gas (N2). For propane fuel the volume of combustion products produced include 3 ft3 of CO2, 4 ft3 of H2O, and 18.87 ft3 of N2. The carbon dioxide and nitrogen gases constitute the inert gases obtained from the flue gases. In addition, nitrogen oxides are also produced and must be removed from the inert gases before injecting them into the underground formations to prevent extensive corrosion of the equipment. The exhaust gases are cooled and washed to remove the combustion water vapors and nitrogen oxides. The clean inert gases of carbon dioxide and nitrogen are then injected into the underground oil formations through existing wells. Following an initial period of injection required for gradually increasing the pressure in the formation, substantial oil either flows or is pumped out through adjacent wells or the injection of the inert gases is discontinued, and oil is allowed to flow back to the well into which the gases were injected when the huff and puff method is applied. The injection of gases into a well and production of oil from an adjacent well is referred to as the “flow through production” method of inert gas production. The injection of gases into a well to increase the pressure in the formation then allowing that pressure in the formation to force the oil to flow back to that same well is referred to as the “huff and puff,” or the “cyclic injection and production” method of inert gas production. In most instances, the specific method used is dependent on the viscosity of the oil being produced.
Saltwater brought to the surface with gas and oil from underground production wells is commonly referred to as “produced water.” The present invention relates to underground production formations where the natural gas has been nearly depleted; therefore, the produced water will be brought to the surface combined with some remaining gas and the oil. The produced oil and water are typically placed into large tanks (often referred to as “gun barrels” in the industry) and allowed to separate by gravity. Although the oil is transported to refineries, the produced water becomes a waste product. However, in the methods employed by the present invention the produced water becomes a valuable commodity to be filtered and injected into the same underground formation from which it originates to act as a blocking mechanism to prevent the injected gases from escaping and direct the flow of the gases driving the oil to the production wells.
Shallow oil producing formations frequently contain oils with higher viscosities than the deeper wells where the volatile products may not generally escape. The viscosity of heavy oils can be reduced by absorption of carbon dioxide (CO2). Where it is determined from laboratory analysis that the reduction of oil viscosity of the oil in the underground formations would be economically beneficial to the production process, carbon dioxide can be separated from the nitrogen gas to nearly 100 percent of the—injection gases to reduce the viscosity of heavy oils. The nitrogen gas can be released to the atmosphere, transported to other oilfields for injection, or used for injection in another part of the oil formation under production as a driving mechanism when the carbon dioxide gas has reduced the heavy oil viscosity. Membranes may be used to separate the nitrogen from the carbon dioxide in flue gases. The membranes are typically employed for production of nitrogen gas with air as the source of nitrogen. The membrane used are assemblies of many thousands of hollow polymeric fibers each approximately the size of a human hair with the inside surface treated to produce a thin film on the inside surface that actually becomes the membrane that allows oxygen molecules to flow through the membrane and reject the larger nitrogen molecules. The porous material below the membrane surface serves as a support. The membranes also allow other gases with molecules smaller than that of nitrogen to flow through and be separated from the nitrogen gas. The result is for pure nitrogen to be separated from all other gases, including water vapors, in atmospheric air. In applications other than oil production the nitrogen is collected and stored, with the gases other than nitrogen typically discharged to the atmosphere. For injection into a heavy oil formation the flue gases can be separated for concentration of carbon dioxide where it is beneficial and economically feasible to do so. The normally discarded gases that flow through the membranes become the product to be collected for injection into the underground heavy oil formation.
The boundaries of the productive formations in existing oil fields were defined in the development and planning phases following the discovery of oil in those areas. The location and spacing of wells on a particular formation was based on the specific structure of the formation and on the number of different operators on the field attempting to achieve maximum oil production. Regardless of how well spacing was originally determined, detailed records of what was accomplished were kept and can be used as a reference to establish a general approach to additional production in specific fields. The structure of the formations, the spacing and location of the wells, and the viscosity of the oil to be produced will determine which wells are selected for inert gas and water injection and the specific method of either flow-through production or cyclic injection and production (huff and puff) from each well.
With the natural gas nearly depleted over the oil in the underground formations where the oil production is to occur methane for engine fuel may not be readily available in the oilfields. The cost of a natural gas (methane) pipeline or the trucking of propane to some of the oilfields may be substantial. In those fields the fuel gas might be economically extracted from the crude oil produced in those oilfields by a fuel gas generator. The gas extracted from the crude oil can be used as fuel for the engines that power compressors, and for other engines that power generators to supply electrical power for pumps, cooling tower fans, controllers, and area lighting where electricity is not readily or economically available, or for competitive cost advantage over other methods of producing oil from nearly depleted formations.
It is estimated that approximately 30 percent (or roughly one-third) of the recoverable oil has been removed from oil reservoirs since the beginning of the oil Industry in the United States. The rest of the oil is still in the ground from which the major oil companies have withdrawn. The formations are left with minor production by a large number of small producers expending a tremendous amount of effort to produce only a fraction of the original production without the natural mechanisms to drive the oil to the production wells where the oil can be lifted to the surface. Some of the remaining oil is highly viscous and only a few hundred feet from the surface. Water and steam have been injected (called “flooding”) into the formations and used with some success to produce oil. The permeability of many reservoirs is not suitable for water or steam flooding. In very shallow reservoirs the formations above the oil reservoirs could not retain the gases (light ends) that make the oil highly fluid and might not be thick enough to retain the water or steam pressures required to drive the heavy oil to the production wells. Under the right conditions carbon dioxide is miscible with oil. Carbon dioxide flooding has been used to mix with and lighten crude oil (lower the viscosity) for production where naturally occurring carbon dioxide is readily available from underground formations. Transporting the carbon dioxide from the mines (or wells) to the oil production reservoirs requires costly cross-country pipelines and large compressors to convey the carbon dioxide. Even with large investments in carbon dioxide wells and pipelines contributing to the costs of using the carbon dioxide in the reservoirs, the technology is used extensively in the Permian Basin oilfields of West Texas and New Mexico.
Certain oil reservoirs contain paraffin-based oils that can be difficult to produce. When a highly paraffin-based oil is subjected to a pressure drop or a decrease in temperature, such as that which can occur as the oil flowing in the reservoir approaches a production well, the paraffin might precipitate (come out of solution) and block the passageways (permeability) in the reservoir sufficiently to prevent the oil from reaching the well. The problems associated with production of paraffin-based oils can prevent removal of a major part of the oil contained in that type of reservoir. What occurs in one particular formation located in East Texas and Northern Louisiana is a good example of the problems associated with paraffin-based oils. There are estimates that suggest less than 15 percent of the recoverable oil has been produced since the discovery of this oil formation, perhaps one of the largest oil deposits in the United States. A well drilled into the reservoir typically produces anywhere from 50 to 150 barrels (42 gallons per barrel) of high-grade crude oil a day when completed. Within a period of perhaps 45 days the oil production starts to decrease, and in a short period of time is reduced to between 2 to 6 barrels a day, with some declining even further. Oil pumps typically have to be pulled out of the wells as often as every 90-days to steam off the accumulated paraffin blocking the oil paths in the pumps and piping to the surface. The dissolved-gas-driven, limestone reservoir is not suitable for extensive water or steam flooding. The difficulty has caused many producers to abandon the fields. Many wells (thousands of them) have been abandoned and are still opened to the atmosphere (called “orphan wells”) waiting to be plugged by the States in which they exist. A temperature increase of only a few degrees (perhaps 2 to 5 degrees) could keep the paraffin from precipitating and keep oil production to the level first encountered when the wells are placed into initial production until the natural drive mechanism within the reservoir is depleted. Many attempts have been made to heat this and other types of oil reservoirs using electrical power without success. Some have been attempted under severe safety hazards. There are stories in the Industry about the heating of some wells being attempted by applying electrical power from high-voltage electrical power lines directly to the oil production well casings (which are grounded) resulting in countywide power blackouts.
Some of those efforts were attempts to pressurize the reservoirs by heating the connate water and create steam to act as a driving mechanism instead of only maintaining the paraffin in liquid form or fluidizing the heavy oils sufficiently to flow through the formation. Formation heating was also attempted by applying the heat in the same well as the oil was to be produced. That prevented the pump from being inserted in the same well during the attempted heating process. As a result, the pressurization attempted was to build up pressure through the production well then try to have the oil flow back to that same well in what is known in the Industry as the “huff-and-puff” method of production. Without a pump, the pressure in the formation had to be high enough to drive the oil all the way to the surface by the underground pressure. This is one of the reasons electrical heating in the past resulted in less than desirable results. By using separate wells to heat around the production well as in the present invention, a “flow-through” method of production with a separate injection drive mechanism (e.g. gas) can be used to drive the oil through the formation at a much lower pressure and have the pump in the production well lift the oil to the surface. This is a significant improvement over what was attempted in the past.
When the oil is trapped in adequately closed reservoirs some of the gases remain in solution within the oil keeping the oil fluid enough to flow through the passageways in the reservoirs to production wells where it is lifted to the surface by either natural gas pressure or pumps. Where the formations above the reservoir do not adequately seal the reservoir, or where the oil reservoirs are exposed to the atmosphere, the light ends (natural gases) escape in many reservoirs, and the remaining oil may become highly viscous and cannot flow through the porous spaces to the production wells. The extremes of that process may cause the oil remaining to become tar-like, called “bitumen.” Some bitumen formations (also referred to as “tar sands”) are so thick (perhaps over 1,000 feet) that it seems the entire ocean basins where the oil originated may have been uplifted to the surface where the gases have escaped by being exposed to the atmosphere over many millions of years or heated during the geological movement to cause the reservoir to expel the gases. Some of the largest oil reserves known are in bitumen formations where oil extraction is extremely difficult and some attempts have been abandoned or placed on hold after investing billions of dollars in production programs when oil production was considered too costly until new technology emerges. Some of the best-known bitumen reservoirs are in Canada, Venezuela, and (a smaller reservoir) the U.S. State of Utah.
Where severe and costly problems of paraffin-based oil production are incurred, or where viscous crude oil production has been historically low with conventional pumping, the present invention can be economically used in many areas to heat the reservoirs adjacent to the production wells. The present invention overcomes the deficiencies of existing systems and methods of oil production, which will become apparent to those skilled in the art having the benefit of the present disclosure, by heating the area adjacent to the production wells and maintaining the temperature of the incoming oil and produced water high enough to keep the paraffin in solution. In highly viscous oil fields the entire oil to be produced might be heated enough to make it flow through the reservoir to production wells where, again, it can be lifted to the surface. The present invention can be used in conjunction with the reservoir drive mechanisms disclosed in co-pending patent application Ser. No. 10/317,009, filed Dec. 11, 2002, entitled “Methods and Apparatus for Increasing Oil Production from Underground Formations Nearly Depleted of Natural Gas Drive,” by Johnny Arnaud and B. Franklin Beard, which is hereby incorporated by reference herein in its entirety.